1. Field of the Invention
The present invention also relates to an optical module and an optical transmission device and, in particular, to an optical module and an optical transmission device for transmitting an optical signal on the basis of positive and negative differential signals or converting a received optical signal into positive and negative differential signals.
2. Description of the Related Art
In the optical communication, a low cost optical module having an optical semiconductor such as a light emitting element or a light receiving element and an optical transmission device utilizing such optical module have been used. An optical transmission device of such kind, in which an optical semiconductor is mounted and sealed on a lead frame and a plurality of leads arranged in one line are derived from the sealed optical semiconductor so that the lead frame can be through-mounted on a circuit board, is known. (see, for example, JP-A-2002-344024 and JP-A-2002-151704).
FIGS. 18(a) to 18(c) show the related art optical module, in which FIG. 18(a) is a front view of the related art optical module 100, FIG. 18(b) is a side view thereof and FIG. 18(c) is a bottom view thereof. The optical module has the SIP (Single In-line Package) type construction, in which leads arranged in one line are derived.
The optical module 100 includes a body portion 101 in which an optical semiconductor and a drive IC, which are not shown, mounted on a lead frame and sealed by a resin package and a plurality of leads 102 arranged in one line and derived from one side of the body portion 101, as shown in FIG. 18(a).
The leads 102 are identical in configuration and length and are regularly arranged in the line at a constant pitch as shown in FIGS. 18(a) and 18(c). In order to through-mount the optical module 100 on the circuit board, through-holes 103 each having diameter slightly larger than an outer size of the lead 102 are provided in the circuit board. Therefore, it is necessary to make pitch p of the leads 102 larger than diameter d, which includes clearance with respect to the diameter of the through-hole 103. That is, pitch p of the leads 102 is limited by the size of the through-hole 103 and it is impossible to provide pitch p smaller than a certain distance. Incidentally, FIG. 18(c) shows an example in which the through-holes 103 cannot be provided.
Recently, compactness and high performance of an optical module have been required, so that it is requested that a number of leads can be derived from the optical module. However, since it is necessary to consider the through-holes 103 in the SIP type optical module as mentioned above, it is difficult to reduce the pitch of the leads 102. In order to solve this problem, the ZIP (Zigzag In-line Package) type optical module is provided.
FIGS. 19(a) to 19(c) show a related art ZIP type optical module 200, in which FIG. 19(a) is a front view thereof, FIG. 19(b) is a cross sectional view thereof and FIG. 19(c) is a bottom view thereof.
This optical module 200 is called as ZIP type optical module having a plurality of zigzag arranged leads. As shown in FIGS. 19(a) and 19(b), the optical module 200 includes a light emitting element 202 such as a laser diode, a drive IC 203 for driving the light emitting element 202, a lead frame 204 on which the light emitting element 202 and the drive IC 203 are mounted, a plurality of leads 201A and 201B including differential signal leads 201A(−) and 201B(+), a sealing member 205 of transparent insulative material, for sealing other portion than the plurality of the leads 201A and 201B and bonding wires 206 for connecting differential signal input terminals of the drive IC to terminals of the leads 201A(−) and 201B(+).
Since the leads 201A and 201B of this optical module 200 are arranged in two lines, it is possible to reduce the pitch p of the leads compared with the optical module 100 having the leads arranged in one line as shown in FIGS. 18(a) to 18(c). Further, since the leads are arranged in the two lines, it is possible to make the optical module 200 hard to fall on a circuit board when the optical module 200 is mounted on the circuit board.
The recent request of compactness and high performance (high operation speed) of the optical module is also spread to low cost optical modules. One of measures for increasing the operation speed is to obtain differential signals having opposite polarities by supplying a modulation signal to a differential circuit and to drive, for example, the light emitting element on the basis of the differential signals.
FIG. 3 shows waveforms of an input signal S for operating the differential circuit included in a circuit portion of the body portion 101 of FIGS. 19(a) to 19(c). The input signal S is composed of a positive differential signal S+ and a negative differential signal S−. The circuit portion, which operates with the paired signals S+ and S−, generates an output signal, which is a difference signal between the differential signals S+ and S−, and the light emitting element 202 is driven by the output signal.
It is ideal that the input signal S contains only signal component without any noise mixed therein. However, there may be a case where noise SN overlaps on the differential signals S+ and S−, as shown in FIG. 3. In such case, in-phase noise SN are cancelled out in the differential circuit operated by the differential signals S+ and S−, so that the noise SN does not appear at an output terminal of the differential circuit. For this reason, the paired differential signals S+ and S− are used.
FIGS. 20(a), 20(b) and 20(c) show flows of the differential signals S+ and S− in the optical module 200 and the circuit board. In FIGS. 20(a) to 20(c), it is assumed that the adjacent two leads 201A(−) and 201B(+) are leads for inputting the differential signals S+ and S−. Further, it is assumed that the optical module 200 is mounted on a circuit board 210 having wiring patterns 211A and 211B, as shown in FIG. 20(b).
One ends of the differential signal leads 201A(−) and 201B(+) are connected to differential signal input terminals (+) and (−) on the drive IC 203, respectively, and the other terminals thereof are connected to the wiring patterns 211A and 211B on the circuit board 210, respectively. When the differential signals S+ and S− are supplied to the wiring patterns 211A and 211B, a positive (+) side current and a negative (−) side current flow to the wiring patterns 211A and 211B and the leads 201A(−) and 201B(+) along directions shown by arrows in FIGS. 20(a), 20(b) and 20(c).
However, in the related art optical module 200 having the lead structure shown in FIGS. 19(a) to 19(c), there is a portion extending from the sealing member 205, in which the transmission routes of the differential signal leads 201A(−) and 201B(+) are different from each other, as shown in FIGS. 20(b) and 20(c).
Therefore, the cancellation effect of noise SN is reduced, so that there are problems that there is a phase difference and that the signal quality and noise characteristics become worse in a frequency band of gigabits or higher. Incidentally, the same problems occur in an optical module constructed with a light receiving element as the light emitting element 202 and a signal processing IC as the drive IC 203.